1. Technical Field of the Invention
The present invention is relates generally to a system for detecting a preceding vehicle which may be employed in such as an automotive anticollision system. More particularly, the invention relates to an automotive radar system which is operable to minimize errors in detection of a preceding vehicle traveling ahead due to another vehicle moving on a different traffic lane or objects such as billboards, road signs, or lamp-posts arranged along a road.
2. Background Art
An automotive anticollision system is well known in the art which detects distance between a controlled vehicle and a preceding vehicle, and modifies the distance to the preceding vehicle to a target value of interest to a driver.
Such an anticollision system includes a radar unit for exact measurement of distance to a preceding vehicle. The radar unit of this type normally uses electromagnetic waves with high directivity such as radio waves or laser beams so that the distance to an object present in front of the controlled vehicle is measured with high accuracy. However, a drawback is encountered in that when the controlled vehicle is moving on a curved road, a vehicle traveling on a different traffic lane as well as billboards or reflectors arranged along the road may be tracked undesirably. This results in anticollision control not being carried out properly, reducing riding comfort and driving stability.
For avoiding the above drawback, Japanese Patent First Publication No. 60-169333 discloses a vehicle speed control system which holds vehicle speed at a fixed rate when a controlled vehicle is traveling on a curved road and releases the vehicle speed holding control when the intervehicle distance detected is less than an allowable minimum distance determined based on a vehicle speed or a steered angle.
In addition, Japanese Patent Second Publication No. 3-78596 teaches a preceding vehicle detecting system wherein electromagnetic waves are radiated forward in different directions, when a preceding vehicle tracked in the first direction moves out of a detectable range over a preselected distance ahead of a controlled vehicle, another vehicle present in the second direction is considered a preceding vehicle the system should track.
However, the above prior art systems have suffered from the following disadvantages.
In the former system, when a distant vehicle moving on a different traffic lane, billboards, or reflectors along a road is tracked while the controlled vehicle is cornering with a smaller steered angle or when the speed of the control vehicle is held at the fixed rate when the controlled vehicle is cornering with a greater steered angle, the reliability of detection of a preceding vehicle present on the same traffic lane as the controlled vehicle becomes low.
Additionally, in the later prior art system, while a preceding vehicle is being tracked with electromagnetic waves radiated in a direction, when another preceding vehicle is sensed by electromagnetic waves radiated in a different direction, the system does not consider it a preceding vehicle. Therefore, when a vehicle appears suddenly in front of the controlled vehicle from a different traffic lane, the system cannot respond quickly. Further, even when a radar-tracked preceding vehicle moves to a different traffic lane while traveling straight, the system may remain tracking the vehicle traveling on the different traffic lane.